In order to improve performance and packing density of modern microelectronic devices, it is often desirable to reduce channel lengths of metal-oxide-semiconductor-field-effect transistors (“MOSFETs”) during device design. As the MOSFET channel length decreases, however, short channel effects and parasitic resistance become of increasing concern. To minimize short channel effects in bulk silicon devices, for example, the source/drain doping junction depths are decreased during scaling. But shallower junctions require the use of thinner silicides to minimize leakage current from the silicon/silicide interface to the junction depletion region, which, at the same time, may increase parasitic contact resistances. As another example, to improve performance of a silicon-on-insulator (“SOI”) device by reducing short channel effects, its silicon region should preferably be less than about 20 nm thick. Conventional silicide formation processes, however, may consume substantially the entire silicon layer of such thickness, which may, in turn, result in undesirably large leakage currents and parasitic contact resistance because the silicide/silicon interface area is small.
The increasing operating speeds and computing power of microelectronic devices have recently given rise to the need for an increase in the complexity and functionality of the semiconductor substrates that are used as the starting substrate in these microelectronic devices. Such “virtual substrates” based on silicon and germanium provide a platform for new generations of VLSI devices that exhibit enhanced performance when compared to devices fabricated on bulk Si substrates. Specifically, new technological advances enable formation of heterostructures using silicon-germanium alloys (hereinafter referred to as “SiGe” or “Si1-xGex”) to further increase performance of the semiconductor devices by changing the atomic structure of Si to increase electron mobility. These substrates are called strained Si substrates.
A strained Si substrate is generally formed by a first epitaxial growth of a relaxed SiGe layer on bulk Si, and then a second epitaxial growth of a thin (less than about 500 Å) Si layer on the relaxed SiGe layer. Because the lattice constant of relaxed SiGe heterostructure is different from Si, the thin Si layer becomes “strained,” resulting in enhanced mobilities (and hence improved device speeds) over bulk Si. The percentage of Ge in SiGe, and the method of deposition can have a dramatic effect on the characteristics of the strained Si layer. U.S. Pat. No. 5,442,205, “Semiconductor Heterostructure Devices with Strained Semiconductor Layers,” incorporated herein by reference, demonstrates one such method of producing a strained Si device structure.
A method of epitaxially growing a relaxed SiGe layer on bulk Si is discussed in international application WO 01/22482, “Method of Producing Relaxed Silicon Germanium Layers,” incorporated herein by reference. The method includes providing a monocrystalline Si substrate, and then epitaxially growing a graded Si1-xGex layer with increasing Ge concentration at a gradient of less than 25% Ge per micron to a final composition in the range of 0.1<x<1, using a source gas of GexHyClz for the Ge component, on the Si substrate at a temperature in excess of 850° C., and then epitaxially growing a semiconductor material on the graded layer.
Another method of epitaxially growing a relaxed SiGe layer on bulk Si is discussed in a paper entitled, “Low Energy plasma enhanced chemical vapor deposition,” by M. Kummer et. al. (Mat. Sci. & Eng. B89, 2002, pp. 288-95), incorporated herein by reference, in which a method of low-energy plasma-enhanced chemical vapor deposition (LEPECVD) is shown, which allows the formation of a SiGe layer on bulk Si at high growth rates (0.6 micron per minute) and low temperatures (500-750° C.).
To grow a high-quality, thin, epitaxial strained Si layer on a graded SiGe layer, the SiGe layer is, preferably, planarized to reduce the surface roughness in the final strained Si substrate. Current methods of chemical mechanical polishing (CMP) are typically used to improve the planarity of surfaces in semiconductor fabrication processes. U.S. Pat. No. 6,107,653, “Controlling Threading Dislocations in Ge on Si Using Graded GeSi Layers and Planarization,” incorporated herein by reference, describes how planarization can be used to improve the quality of SiGe graded layers.
Although the resulting biaxial tensile or compressive strain alters the carrier mobilities in the layers of the “virtual substrate” enabling the fabrication of high-speed and/or low-power devices, short channel effects and parasitic resistance remain of concern for strained-Si-based devices as well.
Raised source/drain structures have been proposed as a technique for forming high-quality contacts to improve performance of bulk silicon, silicon-on-insulator (SOI), and strained silicon devices. Raised source/drains are generally fabricated by raising the level of the source and drain regions by selective semiconductor, e.g., silicon, deposition. The extra silicon increases the process margin for the silicide process and extends the latitude for contact junction design. To maintain a similar crystalline structure, the extra silicon is “grown” by silicon epitaxy.
For example, in bulk and strained Si devices, raised source/drain contacts have been proposed as a means of forming shallow dopant junctions to minimize contact resistances and short channel effects. In SOI devices, the raised source and drain regions provide sacrificial silicon useful for silicide consumption to form low resistance contacts on thin Si films.
Typically, source and drain regions are formed by selective epitaxial silicon growth after formation of a sidewall dielectric spacer of silicon dioxide (SiO2), silicon nitride (Si3N4), or a combination of both materials. In this process, silicon is epitaxially grown on exposed windows in a dielectric mask while nucleation of polysilicon on the masking material is suppressed during the incubation time by, for example, etching of spurious nuclei on the dielectric material by hydrogen chloride, the mediation of saturation by formation of a number of intermediate chlorine-containing silicon precursors, and passivation of surface defect sites which serve as heterogeneous nucleation centers. Also, selectivity is facilitated by growing for a period of time that is generally shorter than the incubation period needed for polysilicon nucleation on the dielectric mask.
The dielectric spacer electrically isolates a gate made of, for example, polysilicon, from the source and drain regions. After the selective epitaxial growth step, a heavy low-energy implant forms a doped region, and is followed by a silicidation process for, e.g., low resistance complementary metal-oxide-semiconductor (CMOS) contacts.
During selective epitaxial growth, however, decreased thickness of the epitaxial layer typically occurs at interfaces between the epitaxial layer and the dielectric spacer or field oxide, where facets form by growth of low-energy crystal planes that minimize the energetically less favorable epitaxial Si/dielectric material interface. Because the raised source/drain regions grow thinner near the dielectric spacer edge, faceting leads to non-planar device layers. This phenomenon raises processing concerns in bulk Si, SOI, and strained Si devices by complicating implementation of selective epitaxial growth in device applications. Specifically, thinner faceted regions cause localized deeper dopant penetration during ion implantation, which increases short channel effects and leakage currents. Also, thinner faceted regions typically result in deeper silicide penetration and greater Si consumption during silicidation. Thus, faceting may result in poor doping profile control and poor silicide depth control, leading to degraded device characteristics and more complex device designs.
For example, referring to FIG. 1, a cross-sectional transmission electron microscope (XTEM) image illustrates undoped Si layers 10 grown adjacent to reactive ion etched <110>-oriented SiO2 sidewalls 12. The SiGe layers 14 were included as markers to track facet development and evolution. The selective Si epitaxy has noticeable (311)-oriented facets 15 and (110)-oriented facets 17 next to SiO2 sidewalls. Such faceting would be detrimental during implant and silicidation of source and drain regions. Referring to FIG. 2, a silicided source region 20 is formed by selective epitaxial growth on a thin film SOI substrate 22 according to any method known in the art. Such faceted growth leads to deep penetration of a silicide 24 to a buried SiO2 layer 26. This penetration may dramatically increase the contact resistance because the silicide/silicon channel interfacial area is reduced, which leads to poor device characteristics. The electrical contact to the device channel is made through the area adjacent to the spacer 28, which is only the width of the Si channel. Furthermore, the contact quality may degrade because of incomplete silicide formation if the thickness of consumed Si is insufficient.
Similarly, in strained-Si-based devices, thinner faceted raised source and drain regions may cause the silicide to penetrate into a relaxed silicon-germanium (SiGe) layer underneath a strained Si layer. This may result in formation of a poor quality silicide because of the rejection of Ge during alloying of the SiGe layer with the silicide metal. This rejection may create high-resistivity regions that compromise the contact quality.
Facetless selective epitaxial growth is, therefore, desirable for fully realizing the performance advantages of the raised source/drain scheme for fabrication of low-resistance contacts and shallow junctions in advanced devices that have wide ranging application to Si, SOI, and strained Si technologies.
Recently, facetless epitaxy has received much attention in the design of raised source/drain contacts for CMOS applications. For example, selective growth in inverse patterns, i.e. using SiO2 pillars with a Si field as opposed to Si windows in a SiO2 field, has been proposed. Another approach involves fabrication of facetless raised source/drain structures by selective epitaxial growth in commercially available low-pressure and atmospheric-pressure chemical vapor deposition systems by carefully controlling the geometry of the multilayer spacer structure and sidewall spacer profile. For example, it has been suggested that undercutting the liner oxide and silicon growth under the silicon nitride spacer recesses the silicon-liner oxide interaction away from the source/drain region. This gives extra thickness in the source/drain region before the increase in silicon-dielectric interface energy causes the epitaxial layer to facet.
In yet another approach, discussed in a thesis entitled “Selective SiGe Nanostructures” by Tom Langdo (Ph.D. Thesis, MIT, 2001)(“Langdo thesis”), incorporated herein by reference, facetless growth was demonstrated by in situ n-type doping of a silicon layer being deposited by selective epitaxy adjacent to a sidewall of a reactive ion etched SiO2 spacer in a ultra-high vacuum chemical vapor deposition (UHVCVD) system. It was demonstrated that a combination of moderate n-type doping and relatively vertical SiO2 sidewalls results in facet-free epitaxy along <110>-oriented SiO2 sidewalls. Referring to FIG. 3, an XTEM image illustrates growth of 1×1018/cm3 n-type doped Si with SiO0.9Ge0.1 marker layers on <110>-oriented SiO2 sidewalls. As shown in FIG. 3, all Si0.9Ge0.1 marker layers follow (100) planes, and thus, facet formation is suppressed during the growth. SiO2 was chosen as the sidewall material instead of more commonly used Si3N4 because selective growth generally cannot be achieved with either SiH4 or SiH2Cl2 source gases, common Si precursors, on Si3N4 spacers without the addition of hydrogen chloride. Because of the deleterious effects of chloride on a UHVCVD system, however, addition of hydrogen chloride to the source gases is impractical.
Obstacles hinder the commercialization of this UHVCVD growth approach. First, UHVCVD is generally not feasible for large-scale commercial applications. Second, UHVCVD selective growth is not possible with Si3N4 dielectric materials. Finally, the UHVCVD process described above is generally not compatible with multi-layered commercial spacer structures that include, e.g., Si3N4 materials.
Thus, there remains a need in the art for a process of fabricating semiconductor devices having facetless raised source/drain structures that is compatible with existing semiconductor manufacturing processes, systems, and materials and that also possesses improved process tolerances.